TypeScript
Property decorator
Learn property decorators.
Property decorator
A property decorator is written right above a class property and takes two arguments: (target, key). target is the class prototype and key is the property name. Unlike a method decorator, no descriptor is provided; the property may not be initialized yet.
function Kayit(
target: any,
key: string
) {
console.log("özellik: " + key);
}
class Kisi {
@Kayit ad: string;
}
Property decorator use cases
Property decorators are typically used to register property names, add validation rules, or carry metadata. At runtime the property value may not be assigned yet; therefore the decorator usually records and processes the value later. For example, a system that auto-discovers form fields can collect field names via property decorators.
Exercise Add property registration
Write the Kayit decorator taking (target: any, key: string) that prints the property name via console.log("özellik: " + key). Write class Kisi with property ad: string and constructor(ad: string) that does this.ad = ad. Apply the decorator manually by calling it with Kisi.prototype and "ad". Assign new Kisi("Ayşe") to a variable k, print k.ad with console.log.
Show example solution
function Kayit(target: any, key: string): void {
console.log("özellik: " + key);
}
class Kisi {
ad: string;
constructor(ad: string) {
this.ad = ad;
}
}
Kayit(Kisi.prototype, "ad");
const k = new Kisi("Ayşe");
console.log(k.ad);
Test yourself
-
How many arguments does a property decorator take?
Show answer
2 — A property decorator takes two arguments: target (prototype) and key (property name). Three arguments belong to method decorators; one to class decorators.
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Property decorator signature
function Kayit(target: any, ____): void {
console.log("özellik: " + key);
}
Show answer
function Kayit(target: any, key: string): void {
console.log("özellik: " + key);
}
-
Property decorator parts
- target
- The class prototype
- key
- The property name
- (target, key)
- Property decorator signature
- console.log(key)
- Printing the property name
Next lesson: Decorator factories
